1. Field of the Invention
The present invention relates to a transmission X-ray analyzer and a transmission X-ray analysis method, which are capable of measuring a transmission X-ray output from a sample through use of a time delay and integration (TDI) sensor.
2. Description of the Related Art
Conventionally, a foreign matter in a sample and density unevenness of elements have been detected by X-ray transmission imaging. As a method of the X-ray transmission imaging, there is known a method of converting a transmission X-ray output from a sample into fluorescent light through a fluorescent screen or the like, and detecting the fluorescent light through use of image pickup devices (charge coupled devices (CCDs)). As a detection method using CCDs, there is a method of scanning a sample to obtain linear images successively through use of a line sensor having a plurality of image pickup devices arranged in one direction, thereby obtaining a two-dimensionally image of the sample.
By the way, when the movement speed of a sample in a scanning direction increases, the time period of accumulation of charge in the line sensor becomes shorter, and in the case where the sensitivity of the line sensor is low, an S/N ratio decreases. For this reason, a time delay and integration (TDI) sensor has been used, in which a plurality of (stages of) line sensors are arranged in parallel in the scanning direction and charge accumulated in one line sensor is transferred to an adjacent subsequent line sensor. In the TDI sensor, the charge accumulated in a line sensor of the first stage is transferred to a line sensor of the second stage. In a line sensor of the second stage, the charge transferred from the line sensor of the first stage is added to the charge accumulated when the line sensor of the second stage receives light, and the resultant charge is transferred to a line sensor of the third stage. Thus, charge transferred from a line sensor of the previous stage is added sequentially to each line sensor, and accumulated charge transferred to a line sensor of the last stage is output.
Accordingly, in the TDI sensor, in the case where the number of stages is T, charge which is T times as large as that of a single line sensor is accumulated, and a contrast becomes T times as high as that of a single line sensor. Further, noise is reduced, measurement can be performed at high speed, and an S/N ratio increases.
On the other hand, due to high sensitivity, the TDI sensor has problems in that a defect (artifact) appears in a detected image due to a change in a received light amount, and that noise is superimposed at a time of rising or falling of a vertical transfer clock for charge transfer. Therefore, a technology of controlling the number of integration stages of the TDI sensor through use of an electric circuit has been developed (Japanese Patent Application Laid-open Nos. 2000-50063 and 2010-4105).
Further, according to the study conducted by the inventors of the present invention, in the case of using the TDI sensor for a transmission X-ray analysis, as the number of integration stages of the TDI sensor increases, a depth of field decreases. In the case of a thick sample, only a part of the sample in a depth direction is focused to be formed as an image, and the remaining part is not formed as an image. Therefore, there is a problem in that the entire sample cannot be grasped.
According to the technology described in Japanese Patent Application Laid-open Nos. 2000-50063 and 2010-4105, however, the number of integration stages of the TDI sensor is controlled through use of an electric circuit, and it is necessary to manufacture a TDI sensor having a dedicated integrated circuit (IC) such as an application-specific integrated circuit (ASIC) and to change arithmetic software of a TDI sensor. Thus, a general-purpose TDI sensor cannot be used, leading to an increase in cost. Further, in the case where the number of integration stages is set on an electric circuit and arithmetic software as in the technology described in Japanese Patent Application Laid-open Nos. 2000-50063 and 2010-4105, it is difficult for a measurer to freely adjust the number of integration stages T depending upon the thickness of a sample and the kind thereof.